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Appendix VI.


in wrought iron, so that screw threads will withstand fracture
when steel threads break at the root. The elongation is more
uniform than that of steel, though of less extent; and the cinder,
as already explained, facilitates welding. Also wrought iron
oxidises less freely than steel, where the manganese is found to
cause pitting.

The principal defect of wrought iron is its transverse weakness,
though this is reduced by the elimination of cinder; while the
real practical objections are its higher cost, and the difficulty of
procuring labour for the puddling.

P. Soo. Aluminium shrinks largely in cooling, and is not
poured therefore directly into the mould, but into what is called
a runner box, from which it flows into the mould when certain
iron plugs are withdrawn. The runners and risers must be
ample in size, and there should be plenty of vents. The
blackening consists of graphite and water. The metal is melted
in crucibles, at a temperature of 1210 F.; and most of the
alloys are treated in the same manner.

As pure aluminium is soft and weak, it is always alloyed
where strength is required. Wolframinium and Romanium are
'light' alloys having up to 10% of other metals, of which the
principal is tungsten, together with some copper and nickel.
Of * heavy' alloys, E3 breaks at 34 tons per sq. in., and has
23% elongation in 2", while R4 has 42 tons breaking and
10% elongation.

Aluminium is rolled cold into very thin plates, but must be
afterwards annealed, and may be forged at a low heat. It must
only be filed with single-cut files to avoid clogging, and must be
cut at high speed with a lubricant of turpentine or petroleum.
Its uses are various, but it is mostly adopted where light weight
is an advantage. It has been tried for the hulls of launches, but
not' with success, because of its rapid corrosion in salt water.
Alloyed with 6% of copper, however, it is not more corrodible
than iron or steel. When added to steel ingots during melting,
in thAproportion of i to 1000, it prevents piping and porousness.